The present invention relates to seal assemblies. It has been found that seal assemblies when used in the exhaust systems of semiconductor manufacturing plants which employ or generate certain corrosive chemicals in the gas stream can deteriorate more rapidly than usual. This is especially true of applications with Fluorine chemistries based on the use of ClF3 and/or NF3. The exhaust process gases attack the main seal element of the seal assemblies which reduces the useful working life of said seal assemblies.
One method of overcoming this problem is to manufacture the main seal element from more inert but more expensive material than the usual elastomeric material.
It is an aim of the present invention to provide a seal assembly for use in a hostile environment such as the exhaust system of a semi-conductor manufacturing plant which has an extended useful service life but does not require the main seal element to be made from an expensive inert material.
In vacuum technology applications it is common practice for seal assemblies to be based on International Standards for the design of vacuum coupling components. This is to allow for interchangeability and to ensure good vacuum engineering practice is followed to produce good performance at low negative (vacuum) pressures. However in many applications for seal assemblies they must also maintain integrity at substantial positive pressures.
It is a further aim of the present invention to provide a seal assembly which meets these diverse requirements.
Another method of overcoming the problem of main seal element attack is to use a metallic or plastics seal element, protected from attack if necessary by suitable corrosion resistant coatings or an elastomeric seal element enclosed or protected by a layer of corrosion resistant material for example, metallic or plastic films or coatings. However, in all such designs, the seal is formed by the metallic or plastic layer in direct contact with the surfaces of the two objects to be sealed. The lack of elasticity of such materials together with the resultant high tightening forces (compression and/or shear) needed with such seal elements requires the use of special clamping means which are stronger, take up more space and are more expensive to manufacture and fit than clamping means suitable for elastomeric seal elements. Such metallic or plastic seal elements also require better surface finishes on the flanges of the objects to be sealed which are more expensive to manufacture and more prone to damage.
It is yet a further aim of the present invention to provide a seal assembly for use in hostile environments such as the exhaust systems of semiconductor manufacturing plants which has an extended useful life but which can be used with the same inexpensive and compact clamping components and methods already in use with elastomeric seals.
According to the present invention, a seal assembly comprises a main O-ring seal element for engaging in a fluid tight manner opposed surfaces of two objects to be sealingly connected together and a barrier element positioned to protect all or a substantial portion of the surface of the main O-ring seal element against contact by the fluid when passing through the seal assembly from one object to the other object.
Preferably, the barrier element is in the form of an O-ring having some elasticity and has an outer surface which engages the inner surface of the main O-ring seal element. The main O-ring seal element may be made from an elastomeric material for example, fluoroelastomer and the barrier element may be made from an inert material such as PTFE.
The seal assembly may include an over-pressure element which will make it suitable for use with systems having high internal pressures. Preferably, the seal assembly includes means for locating accurately the seal assembly between opposed flanges on the respective objects, and controlling the compression of both the main O-ring seal element and the barrier element.